Interactive video subtitle bug

Hi everyone, 

I've made an interactive video and added subtitles to it. Unfortunately, something weird happens. When I click on a navigation hotspot, and go to another part of the video, the original subtitle stays frozen at the bottom of the screen, and the new one appears above it. This happens every time at a navigation hotspot, so there might be five rows of subtitles. I've attached a screenshot to show what I mean.

This happens at Windows - Firefox and Mac - Firefox. It does not happen at Mac - Safari and Mac - Google Chrome. It does seem to be a Firefox specific problem.

Can anyone help me out? Thanks!